Excimer lasers employing KrF or ArF are utilized as light sources for semiconductor lithography apparatuses and corrective devices. Such light sources, however, are known to have disadvantages such as short life spans, toxicity, and poor beam quality.
Research is therefore being directed toward fully solid lasers that are combinations of solid lasers and wavelength conversion elements. Some wavelength conversion elements utilize the nonlinear optic effect of second harmonic generation (SHG), which emits light at twice the frequency of incident light (second harmonic), and there have already been proposed wavelength conversion elements composed of nonlinear optical crystals (see Non-patent document 1, for example).
An outgoing beam from a wavelength conversion element utilizing SHG is a mixture of light of at least two different wavelengths, that of the fundamental wave as the incident beam and that of the second harmonic by SHG. For purposes that require light of a prescribed short wavelength, such as semiconductor lithography apparatuses and corrective devices, it is preferred to allow activity of only the second harmonic, in order to avoid other light effects.
Optical filters are known as a means of selectively extracting light of a specific wavelength. One type of optical filter is a polarized light filter, and for example, Dichronic Color Filte by Thorlabs is well known. However, even when such an optical filter is combined with a wavelength conversion element that accomplishes wavelength conversion of solid laser light in the visible to near-infrared range to emit second harmonic waves from the ultraviolet/vacuum ultraviolet (UV/VUV) range to the visible range, it is often the case that the fundamental wave is not sufficiently removed. Incidentally, the fundamental wave can be removed by combining multiple optical filters or increasing the filter thickness, but this method introduces the problem of significant reduction in second harmonic output.
At the same time, single crystals are being developed as scintillator materials (see Non-patent document 2, for example). According to Non-patent document 2, high-quality CeF3 and NdF3 single crystals can be obtained. However, application of these single crystals to optical parts has not been reported.